The present invention relates to a new and improved construction of a cloth draw-off apparatus for drawing-off cloth woven from warp threads and weft threads in a weaving machine containing a rotatable main shaft.
In its more particular aspects, the present invention specifically relates to a cloth draw-off apparatus for drawing-off cloth or fabric woven with a cloth or fabric fell from warp threads and weft threads in a weaving machine containing a rotatable main shaft and a cloth draw-off roll which is operatively connected via a regulation circuit or regulation means with the main shaft of the weaving machine. The regulating circuit or means comprises a controllable motor which is drivingly connected with the cloth draw-off roll. A regulation circuit arrangement is connected forwardly of the controllable motor in order to control the weft thread set density in the drawn-off cloth as a function of at least the rotational angular position of the main shaft of the weaving machine.
Hitherto known cloth draw-off apparatuses in weaving machines are drivingly connected via transmission means with the main shaft of the weaving machine in order to draw-off the woven cloth in front of the reed of the weaving machine at a predetermined ratio relative to the rotational speed of the weaving machine. Thus the weft thread set per unit length i.e. the weft thread density of the cloth is determined by means of the draw-off rate. Generally, the gear ratio of the known transmission means can be adjusted continuously or in relatively small steps. Apparatus of this type, then, permits the operation at a pre-adjusted gear ratio at all rotational speeds of the weaving machine and thus there should theoretically result the same weft thread set or weft thread density at all rotational speeds.
In practice, however, particularly during a weaving operation using warp and weft material which renders well recognizable a change in the weft thread set or density in the finished cloth, it is noted that, for reasons determined by the technical properties of the weaving machine and by the textile properties of the warp and weft material, significant problems relating to variations in the weft thread density appear due to the use of pre-adjusted transmission means during shut-down and start-up of the weaving machine.
Such variations in the weft thread density may occur for different reasons, for example, due to an elongation or stretching of the warp threads, particularly warp threads made of an elastic material, during shut-down of the weaving machine. In such case the cloth fell of the cloth including the last beat-up weft thread, is moved away from the reed under the tension exerted by the warp threads during shut-down or standstill of the weaving machine. Consequently, at least the first weft thread after restarting the weaving machine is beaten-up against the cloth or fabric fell at a greater distance than the distance which is pre-adjusted by means of the gear ratio of the transmission means between the main shaft of the weaving machine and the cloth draw-off roll. As a result, so-called expanded regions are formed.
The different dynamic behavior of the weaving machine, for example, during start-up, further results in variations of the weft thread density, for example, due to changes in the distance of the reed from the cloth or fabric fell which leads to so-called start-up streaks.
Very frequently, both of the aforementioned defects appear conjointly.
In order to counteract such defects, various measures have been taken at the known cloth draw-off apparatus.
For example, during the first weft insertion after starting up the weaving machine, the cloth draw-off operation is decoupled from the operation of the weaving machine and the weft thread is not inserted. In this manner it is achieved that the weaving machine runs at full rotational speed during the second weft insertion and thus the dynamic behavior of the weaving machine corresponds to the behavior at full rotational speed. In order to also compensate for the formation of the expanded regions, the transmission means then, during the second weft insertion, must be engaged with a delay of an amount corresponding to the extent of the elongation or stretching of the warp threads.
In another known method the cloth draw-off transmission or the cloth or fabric fell is reset prior to the start of the weaving machine by an amount required in order to compensate for the aforementioned elongation or stretching of the warp threads as well as the variant dynamic behavior of the weaving machine during the first weft insertion. In the case that this reset is intended to be automatically effected, the cloth draw-off transmission must be decoupled prior to the start-up of the weaving machine and re-engaged using controlled drive means. When this function is intended to be manually performed, high reliability of the operating personnel is required.
Transmission means of the aforementioned type additionally also permit, to a certain extent, variations in the weft thread density, particularly when such transmission means comprise worm-gear arrangements driven by means of a ratchet wheel which contains a ratchet capable of performing oscillating movements and which ratchet can be lifted-off from the ratchet wheel for the special case of an increased weft thread density.
It is readily seen that these previously described means for affecting the weft thread density at the drawn-off cloth for compensating or preventing the formation of expanded regions and start-up streaks as well as for intentionally varying the weft thread density, are very expensive and still do not satisfy present requirements.